1. Field of the Invention
The present invention relates to a ferroelectric memory element having a high polarization moment and a high Curie temperature, in particular to a ferroelectric memory element having a high polarization moment and a high Curie temperature when a ferroelectric recording layer is made into a thin film of less than 50 nm thickness, and relates to an electronic apparatus provided with this ferroelectric memory element.
2. Description of the Related Art
Currently, the use of ferroelectric materials comprising transition metal oxides, typically represented by PbZr1xe2x88x92xTixO3 (PZT) and SrBi2Ta2O3 (SBT), in the recording layer of a non-volatile memory element has allowed the realization of high memory capacity and low power consumption of such memory devices. A typical elemental structure is produced by laminating a ferroelectric material such as PZT or SBT on top of the lower electrode made of such materials as Pt, Ir, and the like and further laminating such materials as Pt or Ir as the upper electrode to form a capacitor device. It is desirable for such memory elements to have characteristics of high polarization moment and Curie temperature. As requirements of the sensor amplifier, the polarization moment should be at least 10 xcexcC/cm2. Also, from the viewpoint of reliability of recorded data, it is desirable that the Curie temperature be higher than 200xc2x0 C. Further, it is necessary that a structural phase transition does not occur within the temperature range of usage (for example, xe2x88x9210 to 100xc2x0 C.).
In recent years, on the other hand, further refinement in the size of the elemental structure is being made for the purpose of producing memories of gigabit capacity. To realize such an objective, ferroelectric materials must also be made available in thin films. For example, a film thickness of 150 nm has been sufficient when the design rule was 0.35 xcexcm, but when the design rule becomes finer than 0.10 xcexcm, a film thickness of 50 nm or even 20 nm is needed. It has, therefore, become necessary to provide a ferroelectric material that can provide high performance even in such thin film regions.
In thin film regions of less than 50 nm thickness, it is possible to epitaxially deposit a ferroelectric material on the lower electrode. According to a publication in the Japanese Journal of Applied Physics, Vol. 38, (1999), pp. 5305, it is reported that a higher polarization moment and Curie temperature than those in bulk crystals are obtained in the recording layer by growing a tetragonal crystal of BaTiO3 oriented in a  less than 001 greater than  direction. The film thickness in this case is 12 nm. The lower electrode comprises SrRuO3 deposited on the (001) surface of a SrTiO3 substrate. In this case, the length of the a-axis of tetragonal BaTiO3 is compressed by the epitaxial growth so as to match the a-axial length 391 pm of SrTiO3. In this case, to maintain the Poisson""s ratio, the c-axial length of BaTiO3 is elongated even up to 437 pm. The polarization moment of BaTiO3 having the c-elongated crystal structure is about 35 xcexcC/cm2, and the Curie temperature is estimated to be higher than 350xc2x0 C. On the other hand, in a bulk BaTiO3 material, the polarization moment is 29 xcexcC/cm2 and the Curie temperature is 120xc2x0 C. Therefore, the technique of crystal lattice elongation caused by epitaxial growth in a thin film region is an extremely useful technique for raising the polarization moment and Curie temperature. In particular, it can be said that BaTiO3 has become a strong candidate for a ferroelectric memory material because of its ability to significantly increase its Curie temperature.
However, the trend towards high density, high capacity ferroelectric memory elements indicates that an even higher polarization moment than that achievable by the conventional technology is required. The reason is that, as the memory element becomes finer and the area of the capacitor decreases, a ferroelectric material comprising the memory layer is required to generate an even higher polarization moment in order to secure a charge level for activating the sensor amplifier.
A first object of the present invention is, therefore, to provide a ferroelectric memory element having a ferroelectric recording layer that can generate a high polarization moment and a high Curie temperature.
The next consideration relates to the epitaxial growth technique for improving the properties of BaTiO3. As described in the description of the prior technologies, it is first necessary to form an oriented lower electrode comprising a perovskite-type oxide. However, because most ferroelectric memory elements are formed on an amorphous oxide, especially SiO2, it has been difficult to form an oriented lower electrode having a perovskite-type oxide. For this reason, it has been a topic of investigation to find a method for producing an oriented film comprising a perovskite-type oxide even on an oxide material such as amorphous SiO2 or SiOF. Further, another topic of investigation has been to use, in the recording layer, BaTiO3 that has a trigonal crystal structure and that generates a higher polarization moment and a higher Curie temperature than those in a bulk material of BaTiO3. Therefore, it is a second object of the present invention to provide a ferroelectric memory element having a ferroelectric recording layer comprising an electrode of an oriented film of a perovskite-type oxide that can be formed on an amorphous oxide, and a BaTiO3 recording layer of a trigonal crystal structure formed by an epitaxial growth technique on top of the oxide layer, thereby resulting in a ferroelectric memory element having a ferroelectric recording layer that can generate a high polarization moment and a high Curie temperature originating from the extraordinarily high stresses caused by epitaxial growth.
Further, it is desirable to guarantee reliability of the memory element, especially a repeatability of over 1012 cycles. Therefore, a third object of the present invention is to provide a ferroelectric memory element having an electrode comprising an oriented film of a perovskite-type oxide that can be formed on an amorphous oxide, and a BaTiO3 recording layer of a trigonal crystal structure formed by an epitaxial growth technique on top of the oxide layer, thereby resulting in a ferroelectric memory element having a ferroelectric recording layer that not only can generate a high polarization moment and a high Curie temperature originating from the extraordinarily high stresses caused by epitaxial growth, but can also provide a high reliability concerning the reading and writing repeatability.
A fourth object of the present invention is to provide an electronic apparatus which has a ferroelectric memory element such as described above.
Therefore, a first aspect of the present invention relates to a ferroelectric memory element having a recording layer comprising BaTiO3 having a trigonal crystal structure and oriented in a  less than 111 greater than  direction in a pseudo-cubic system so as to utilize a polarization moment in the  less than 111 greater than  direction in a pseudo-cubic system for reading recorded signals.
Accordingly, a high polarization moment and a high Curie temperature are obtained in the recording layer.
In the ferroelectric memory element, the recording layer is formed preferably at a thickness of 50 nm or less.
Accordingly, the memory element in the first aspect can respond to the demand of the design rule of microelements of less than 0.10 xcexcm in the thin film layer of the ferroelectric recording layer.
In the ferroelectric memory element in the first aspect, it is preferable that a unit cell of BaTiO3 crystal structure is elongated in the  less than 111 greater than  direction in a pseudo-cubic system, compared with a unit cell of a trigonal crystal structure of a bulk material of BaTiO3 in a most stable state.
Accordingly, a high polarization moment and a high Curie temperature can be obtained in the recording layer.
In the ferroelectric memory element in the first aspect, it is preferable that a degree of elongation is not less than 2%.
Accordingly, a high polarization moment and a high Curie temperature are obtained in the recording layer.
In the ferroelectric memory element in the first aspect, it is preferable that a thickness of the recording layer is not more than 50 nm.
Accordingly, the memory element can respond to the demand of the design rule of microelements of less than 0.10 xcexcm in the thin film layer of the ferroelectric recording layer.
A second aspect of the present invention relates to a ferroelectric memory element comprising a first electrode layer; a recording layer formed on top of the first electrode layer by laminating BaTiO3 having a trigonal crystal structure in such a way so as to be oriented in a  less than 111 greater than  direction in a pseudo-cubic system and to elongate a unit cell of the trigonal crystal structure in the  less than 111 greater than  direction in a pseudo-cubic system to a level of not less than 2%, compared with a unit cell of a trigonal bulk material of BaTiO3 in a most stable state; and a second electrode layer laminated on the recording layer, thereby allowing utilization of a polarization moment in the  less than 111 greater than  direction in a pseudo-cubic system of the recording layer for reading recorded signals.
Accordingly, a high polarization moment and a high Curie temperature are obtained in the recording layer.
In the ferroelectric memory element in the second aspect, it is preferable that a thickness of the recording layer is not more than 50 nm.
Accordingly, the memory element can respond to the demand of the design rule of microelements of less than 0.10 xcexcm in the thin film layer of the ferroelectric recording layer.
In the ferroelectric memory element, it is preferable that the first electrode layer and the second electrode layer comprise SrRuO3. The first electrode layer and the second electrode layer may also comprise SrTiO3 converted to an electrically conductive form.
By so doing, it is possible to elongate a unit cell in a  less than 111 greater than  direction in a pseudo-cubic system compared with a unit cell of a trigonal bulk material of BaTiO3 in a most stable state, and to achieve a level of elongation of not less than 2%.
A third aspect of the present invention relates to a ferroelectric memory element comprising a first electrode layer comprising a metallic material having a cubic system and oriented in a  less than 111 greater than  direction; a second electrode layer, having a perovskite-type crystal structure and formed on the first electrode layer so as to be oriented in a  less than 111 greater than  direction in a pseudo-cubic system; a recording layer comprising BaTiO3 having a trigonal system and laminated on top of the second electrode layer so as to be oriented in a  less than 111 greater than  direction in a pseudo-cubic system; and a third electrode layer laminated on top of the recording layer, thereby allowing utilization of a polarization moment generated in the  less than 111 greater than  direction in a the pseudo-cubic system in the recording layer for reading recorded signals.
The structure described above allows the formation of an electrode of a perovskite-type structure oriented in a given direction even on an amorphous oxide substance, thereby allowing construction of a ferroelectric memory element having a recording layer, formed on top of the electrode layer, that provides a high polarization moment and a high Curie temperature. In particular, it allows the use of BaTiO3 in a trigonal system that produces a high polarization moment in the recording layer.
In the ferroelectric memory element in the third aspect, it is preferable that a unit cell of BaTiO3 crystal in the recording layer is elongated in the  less than 111 greater than  direction in a pseudo-cubic system compared with a unit cell of a trigonal bulk material of BaTiO3 in a most stable state.
The structure described above allows the formation of an electrode of perovskite-type structure oriented in a given direction even on an amorphous oxide substance, and a ferroelectric memory element can be provided in which a recording layer having a high polarization moment and a high Curie temperature is formed on the electrode on account of a powerful stress created by epitaxial growth on the electrode. In particular, this technique allows the use, in the recording layer, of BaTiO3 in a trigonal system that provides a relatively higher polarization moment.
In the memory element in the third aspect, it is preferable that the first electrode layer comprises Pt having a face-centered-cubic structure. This is because it facilitates forming of a second electrode layer of a perovskite-type structure oriented in a pseudo-cubic system  less than 111 greater than  direction to produce superior memory performance.
In the ferroelectric memory element in the third aspect, it is preferable that the first electrode layer comprises Pt and the second electrode layer comprises SrRuO3. By so doing, it is possible to form a high quality second electrode layer of SrRuO3 and to form a high quality recording layer comprising BaTiO3 having a trigonal crystal structure and oriented in a  less than 111 greater than  direction in a pseudo-cubic system to produce a high performance memory element.
In the ferroelectric memory element in the third aspect, it is preferable that the first electrode layer comprises Pt and the second electrode layer comprises SrTiO3 converted to an electrically conductive form. In such a case also, it is possible to form a high quality SrTiO3 second electrode layer and to form a high quality recording layer comprising BaTiO3 having a trigonal crystal structure and oriented in a  less than 111 greater than  direction in a pseudo-cubic system on top of the second electrode layer to provide a high performance memory element.
In the ferroelectric memory element in the third aspect, it is preferable that the first electrode layer is laminated on top of an amorphous oxide layer. According to the third aspect, the technique permits the formation of a first electrode layer, comprising a metallic material of a tetragonal structure and oriented in a  less than 111 greater than  direction, on the amorphous oxide layer; the formation of a second electrode layer on top of the first electrode layer, comprising a perovskite-type structure and oriented in the  less than 111 greater than  direction in a pseudo-cubic system; and the formation of a recording layer, comprising BaTiO3 having a trigonal crystal structure and oriented in the  less than 111 greater than  direction in a pseudo-cubic system, or the formation of a recording layer comprising BaTiO3 having a trigonal system and oriented in the  less than 111 greater than  direction in a pseudo-cubic system and also elongated in the  less than 111 greater than  direction in a pseudo-cubic system. Such a ferroelectric memory element provides a high performance.
A fourth aspect of the present invention relates to a ferroelectric memory element comprising a first electrode layer comprising Ir or an Ir oxide; a second electrode layer, comprising Pt having a face-centered-cubic structure and formed on the first electrode layer so as to be oriented in a  less than 111 greater than  direction; a third electrode layer, having a perovskite-type crystal structure and formed on the second electrode layer so as to be oriented in a  less than 111 greater than  direction in a pseudo-cubic system; a recording layer, comprising BaTiO3 having a trigonal crystal structure and formed on top of the third electrode layer so as to be oriented in a  less than 111 greater than  direction in a pseudo-cubic system; and a fourth electrode layer, having a perovskite-type crystal structure and laminated on top of the recording layer so as to be oriented in a  less than 111 greater than  direction in a pseudo-cubic system; wherein a unit cell of a crystal structure of BaTiO3 in the recording layer is elongated in the  less than 111 greater than  direction in a pseudo-cubic system compared with a unit cell of a bulk material of BaTiO3 in a trigonal system which represents the most stable state, thereby allowing utilization of a polarization moment generated in the  less than 111 greater than  direction in a pseudo-cubic system in the recording layer for reading recorded signals.
According to the structure described above, it is possible to form an electrode of a perovskite-type structure oriented in a given direction, even on an amorphous oxide substance, and to form a recording layer on top of the electrode layer to produce a high polarization moment and a high Curie temperature to provide a high performance ferroelectric memory element. In particular, it becomes possible to use BaTiO3 in a trigonal system having a relatively large polarization moment. Also, the electrode layer comprising Ir acts as a barrier layer for the diffusion of oxygen so that the reliability of performance of the memory element over repeated recording cycles is improved.
In the ferroelectric memory element in the fourth aspect, it is preferable that the third electrode layer comprises SrRuO3. This technique allows the formation of a third electrode layer of high performance using SrRuO3 and the formation of a high performance recording layer comprising BaTiO3 having a trigonal crystal structure on top of the SrRuO3 electrode layer that is oriented and elongated in a  less than 111 greater than  direction in a pseudo-cubic system.
In the ferroelectric memory element in the fourth aspect, it is preferable that the third electric layer comprises SrTiO3 converted to an electrically conductive form. In such a case also, it is possible to form a SrTiO3 third electrode layer and to form a recording layer comprising BaTiO3 oriented in a  less than 111 greater than  direction in a pseudo-cubic system and elongated in the direction.
In the ferroelectric memory element in the fourth aspect, it is preferable that the first electrode layer is laminated on top of an amorphous oxide layer. This technique in the fourth aspect allows the formation of a first electrode layer comprising Ir or an Ir oxide on top of an amorphous oxide substance, the formation of a second electrode layer comprising Pt having a face-centered-cubic structure and oriented in the  less than 111 greater than  direction on the first electrode layer, the formation of a third electrode layer comprising a perovskite-type structure and oriented in the  less than 111 greater than  direction in a pseudo-cubic system on top of the second electrode layer, the formation of a recording layer comprising BaTiO3 having a trigonal crystal structure and oriented in a  less than 111 greater than  direction in a pseudo-cubic system and elongated in a  less than 111 greater than  direction in a pseudo-cubic system, and the formation of a fourth electrode layer comprising a perovskite-type structure and oriented in a  less than 111 greater than  direction in a pseudo-cubic system on top of the recording layer, thereby providing a high performance ferroelectric memory element.
In the ferroelectric memory element in the fourth aspect, it is preferable that the fourth electrode layer is overlaid with a fifth electrode layer comprising Ir or an Ir oxide. By so doing, resistance to reduction of the recording layer is improved so that the margin of the semiconductor manufacturing process is widened.
In a fifth aspect of the present invention, an electronic apparatus is provided which has the above-described ferroelectric memory element. Since this electronic apparatus has a superior memory element such as described above, it is compact, and has recording data that is highly reliable.
Summarizing the features of the present invention, the invention provides a high performance ferroelectric memory element by fabricating a recording layer comprising BaTiO3 having a trigonal crystal structure and oriented in a  less than 111 greater than  direction in a pseudo-cubic system, thereby allowing utilization of the polarization moment produced in the  less than 111 greater than  direction in a pseudo-cubic system for reading recorded signals. The present invention can thus provide a ferroelectric memory element having a high polarization moment and a high Curie temperature in a thin region in which the film thickness of the recording layer comprising a ferroelectric material is 50 nm or less and an electronic apparatus provided with this ferroelectric memory element.